Hydrogen peroxide has been used for a relatively long time as an oxidizing agent in organic synthesis reactions, and also as a bleach, for example for the bleaching of paper or textiles. The disadvantage of hydrogen peroxide is the tendency toward decomposition into water and oxygen, where, in particular, traces of metals or metal salts increase the rate of the decomposition reaction catalytically. This problem arises with oxidation reactions which use a metal catalyst, in particular a transition metal catalyst, or an enzyme with catalase activity. This means that enzyme- or metal-catalyzed oxidation reactions of this type have hitherto required a large excess of hydrogen peroxide, resulting, in turn, in negative effects with regard to process economics and yield. In addition, the possibility of a xe2x80x9cscale-upxe2x80x9d of such reactions is severely limited, which is associated with undesired, extremely exothermic hydrogen peroxide decomposition with the formation of large amounts of oxygen. In addition, the decomposition of the hydrogen peroxide during the preparation, storage or transportation of hydrogen peroxide or of aqueous solutions thereof, or in other application fields for hydrogen peroxide, such as, for example, in the bleaching of paper and textiles, likewise presents problems.
For these reasons, a very wide variety of additives for stabilizing hydrogen peroxide and aqueous hydrogen peroxide solutions has already been investigated. These are, for example, inorganic salts, such as phosphates, pyrophosphates or stannates, organic compounds, such as organic chelating agents or organic acids. These additives are described inter alia in xe2x80x9cHydrogen Peroxidxe2x80x9d [sic], Schumb et al., published by Reinhold Publishing Company, New York (1955), pages 447 to 539. These additives are in most cases unsuitable for enzyme- or metal-catalyzed oxidation reactions using hydrogen peroxide since they lead to poisoning of the catalyst used.
For example, it is known, from D. de Vos, T. Bein, Chem. Comm. 1996, 917 or D. de Vos, T. Bein, J. Organomettal. Chem. 1996, 520, 195, that acetone as solvent suppresses the hydrogen peroxide decomposition during catalytic oxidation reactions. However, if acetone is used, there is a danger that highly explosive products, such as 3,3,6,6-tetramethyltetroxane, which can precipitate out in the form of explosive crystals during the oxidation or the work-up, form. For this reason, the combination of acetone or another ketone as solvent and hydrogen peroxide is unsuitable for processes on an industrial scale.
Accordingly, it was an object of the present invention to find new additives which ensure the stabilization of hydrogen peroxide primarily in enzyme- or metal-catalyzed reactions to a high degree and which are suitable for industrial scale.
Unexpectedly, this object was achieved by using xcex1-ketocarboxylic esters or aldehydecarboxylic esters. It was also expected that such xcex1-keto- or aldehyde-carboxylic esters only have to be added in small amounts, while, in contrast thereto, acetone or other ketones have to be used as solvents in order to effectively suppress the H2O2 decomposition.
The invention therefore provides for the use of one or more xcex1-keto- and/or aldehydecarboxylic esters for stabilizing hydrogen peroxide in enzyme- or metal-catalyzed oxidation reactions in organic synthesis.
According to the invention, hydrogen peroxide in enzyme- or metal-catalyzed oxidation reactions is stabilized by adding a sufficient amount of xcex1-keto- and/or aldehydecarboxylic esters, and suppresses the decomposition of hydrogen peroxide into water and oxygen. Furthermore, this is likewise of importance, for example, in the preparation, storage or transportation of hydrogen peroxide or of hydrogen peroxide solutions, where even small amounts of metal impurities can lead to decomposition.
The invention further provides, accordingly, for the use of one or more xcex1-keto- and/or aldehydecarboxylic esters for stabilizing hydrogen peroxide or hydrogen peroxide solutions in the preparation, storage or transportation.
Hydrogen peroxide or solutions thereof stabilized in this way are suitable not only for enzyme- or metal-catalyzed oxidation reactions in organic synthesis, they can, for example, also be used for the bleaching of paper and textiles, such as, for example, linen, cotton, wool, silk or jute.
Suitable xcex1-keto- or aldehydecarboxylic esters are all xcex1-keto- or aldehydecarboxylic esters with the structural element of the formula I 
Here, R1 and R2 in the case of xcex1-ketocarboxylic esters are a saturated or unsaturated, branched, unbranched or cyclic C1-C30-alkyl radical or an aromatic or heteroaromatic radical, where these radicals may be unsubstituted or substituted by C1-C30-alkoxy, amino, amide, cyano, carbonyl, halogen, hydroxyl or nitrile groups.
In the case of the aldehydecarboxylic esters, R2 is hydrogen. R1 has the same meaning as defined above.
Preferably, in the case of xcex1-ketocarboxylic esters, R1 and R2 are a C1-C5-alkyl radical, such as methyl, ethyl, propyl, isopropyl, tert-butyl or a benzyl radical. The methyl radical is particularly preferred. In the case of aldehydecarboxylic esters, R1 is likewise preferably a C1-C5-alkyl radical, such as methyl, ethyl, propyl, isopropyl, tert-butyl or a benzyl , radical, and particularly preferably a methyl radical. R2 in this case is hydrogen.
Thus, according to the invention, particularly preferred stabilizers are methyl glyoxylate or methyl pyruvate.
The xcex1-carbonyl esters can also be used as stabilizers in the form of a corresponding hemiacetal or full acetal.
Suitable glyoxylic ester hemiacetals are described, for example, in EP-P-0 099 981. Preference is giving to using glyoxylic methyl ester methyl hemiacetal (GMHA), glyoxylic ethyl ester hemiacetals, glyoxylic propyl ester hemiacetals, glyoxylic isopropyl ester hemiacetals, glyoxylic t- or n-butyl ester hemiacetals. Particular preference is given to using GMHA as hemiacetal.
Suitable full acetals are dialkyl acetals, such as dimethyl acetal.
For the stabilization it is possible here to use only one xcex1-keto- or aldehydecarboxylic ester, and also mixtures of two or more xcex1-ketocarboxylic esters, two or more aldehyde carboxylic esters or mixtures of xcex1-keto- and aldehydecarboxylic esters.
The stabilizer according to the invention or a stabilizer mixture is used here in an equivalent amount, in an excess or in a substoichiometric amount relative to the hydrogen peroxide. Preference is given to adding 0.05 to 1.5 mol, particularly preferably 0.2 to 1.2 mol, of stabilizer or stabilizer mixture per mole of hydrogen peroxide. The stabilizer can, for example, be used in a substoichiometric amount if hydrogen peroxide is added slowly during the catalytic oxidation reaction. If this is not the case, then the addition of an equimolar amount of stabilizer is very particularly preferred.
The stabilizers listed above are suitable for stabilizing hydrogen peroxide in enzyme- or metal-catalyzed oxidation reactions. In oxidation reactions, the reaction solutions comprise at least the substrate to be oxidized, an organic solvent or solvent mixture suitable for the oxidation reaction in question, and a metal catalyst, preferably a transition metal catalyst, or an enzyme catalyst.
The reaction solution may optionally comprise a buffer solution.
The stabilizers according to the invention are, accordingly, used in catalyzed oxidation reactions, irrespective of the nature of the catalyst used and of the solvent used.
Catalysts which may be present are, accordingly, for example all customary oxidation catalysts, such as, for example, RuO2, [Mn2O3{Me3tacn}2](PF6)2, (Me3tacn=1,4,7-trimethyl-1,4,7-triacyclononane), V2O5, manganese(II) acetate, chromium(III) nitrate etc., or enzymes, such as vanadium haloperoxidase etc.
Solvents may, for example, be alcohols, such as methanol, ethanol, t-butanol, etc., ethyl acetate, acetic acid, acetone, acetonitrile, methylene chloride, chlorobenzene etc. and water.
This is of great importance particularly for those oxidation reactions in which hitherto a large excess of hydrogen peroxide has had to be used, which has a negative effect on the economic suitability and safety of the process, the reaction volume and on the possibility of xe2x80x9cscale upxe2x80x9d, based on the undesired, extremely exothermic decomposition of hydrogen peroxide in combination with the formation of large amounts of oxygen.
Preference is given to using the stabilizers according to the invention in epoxidations, alcohol oxidations, oxidations of Cxe2x80x94H bonds, oxidative halogenations etc.
In addition, the stabilizers according to the invention can also be used for stabilizing hydrogen peroxide during its preparation, storage or transportation, or in the case of its use as bleach for paper and textiles.
Hydrogen peroxide can also be in the form of an aqueous or an organic solution.